Balo's Bolster Dead

Necrotic energy flows out from you, bolsterin your undead allies. Each friendly undead within 30 feet of you gains temporary hit points equal to twice your spellcasting ability modifier for 8 hours. If you expend your life force when you cast this spell, you can increase the temporary hit points each target gains. Expend and roll a number of Hit Dice of your choice up to half your maximum Hit Dice. Each target of this spell increases the temporary hit points it gains by that amount.

Hit Dice spent in this way can't be used to regain hit points during a short rest. You regain spent Hit Dice as normal.